Magnetic data storage devices



July 30, 1963 E. J. WILLIAMS 3,099,822

MAGNETIC DATA STORAGE DEVICES Filed Sept. 22, 1959 2 Sheets-Sheet 1READNG AMPUFIER AMPLIHER 20 2/-FILTER CUPPER l I 3/ I 22 CLPPER PHASE IPUSH-PULL ZEKC/S SPUTTER AMPUHER *OUTPUT Y Tamsmmnq F\LTER 34 250 KC/&

/5 23- OUTPUT Y TRAN FORMER l6 cuPPER 33 AMPUHER o D\V\DER & q G W,TRANsromm HQ 2 27 2a 29 24 \NvEN'roR EHR/S JED/WV MAL fins ATTORNEY-SJuly 30, 1963 E. J. WILLIAMS MAGNETIC DATA STORAGE DEVICES 2Sheets-Sheet 2 Filed Sept. 22, 1959 'HSOV \NVENI'OR Em? rs J'amv HumansBY AQMQ, ML A'r'rokNevs United States Patent Office 3,099,322 PatentedJuly 30, 1953 3,099,822 MAGNETIC DATA STORAGE DEVICES Emrys JohnWilliams, Stevenage, England, assignor to In- The present inventionrelates to methods of producing magnetic storage devices, such forexample, as magnetic drum or disc storage devices, which have aso-called clock track recorded on them. The present invention alsorelates to apparatus for use in the production of such magnetic storagedevices.

The provision of one or more clock tracks on a magnet-ic storage deviceof the kind in which signal elements, for example, binary signalelements or bit, are recorded at corresponding uniformly spacedpositions along the lengths of a plurality of signal tracks, is wellknown. The clock tracks are such that, in operation of the device,apparatus associated with reading heads which are aligned with the clocktracks, can produce timing signals for use firstly in recording signalelements at the required positions on the signal tracks and secondly,when reading signals from the device, in indicating all the instants oftime at which signal elements may be read from the signal tracks.Commonly, on a drum or disc storage device, two clock tracks areprovided, one for producing a timing signal for every signal elementposition along the signal tracks and the other for producing a timingsignal once per cycle of operation of the device. In some cases a thirdtrack is also provided for producing a timing signal marking thebeginning of each of a plurality of groups of signal elements, i.e.,word position signals.

Broadly speaking two methods have so far been used for the production ofa clock track for use in the production of timing signals correspondingone to each signal element position on the signal tracks. In one case,suitable signals have been recorded on the device under the control of'an oscillator of suitable frequency. In the other physical marks aremade on the device or a storage surface of the storage device, forexample, by use of a dividing head or by a photo-etching technique.These marks may be made in the magnetic part of the surface or on anon-magnetic part of the surface alongside the magnetic part, forexample, on a non magnetic In the latter case the marks are subsequentlyfilled with magnetic material to enable the desired signals to bederived by means of a magnetic reading head and associated apparatus.

Clock tracks produced by both these methods, although enabling therequired number of timing signals e.g. clock pulses, to be produced,suiier from the disadvantage that small variations in the clock trackare apparently unavoidable, with the result that the timing signalsderived from it in operation may have spurious amplitude and/ orfrequency modulation. These spurious efiects are more pronounced, thehigher the repetition frequency of the timing signals, this will behigher, the closer the signal element position spacing along -a trackand/ or the higher speed of scanning of the tracks, and may lead tounacceptable inaccuracies where the repetition frequency is of the orderof 250 kc./s. or more.

It is an object of the present invention to provide a method ofproducing magnetic storage devices which have a clock track recorded onthem, the timing signals derived from which in operation are relativelyfree from spurious modulation.

According to the present invention a method of producing a magneticstorage device which has a clock track recorded on it, comprises a firststep of recording a primary clock track on the storage device and asecond step of recording a secondary clock track on the storage device,the signals recorded to form the secondary clock track being produced byreading the primary clock track and submitting the signals derived by sodoing to amplitude limitation and frequency filtering such as to reduceany spurious amplitude land/or frequency modulation of these signals.

The second step may be repeated as many as may be considered desirable,the signals recorded at each repetition being produced by reading thesecondary clock track already recorded and submitting the signalsderived by so doing to amplitude limitation and frequency filtering suchas to reduce any spurious amplitude and/or frequency modulation of thosesignals.

In the case of a rotary magnetic storage device such as a magnetic drumor disc storage device, the primary clock track may be recorded on it byrecording signals derived from a signal generator which is mechanicallysynchronised to the rotation of the storage device. The signal generatormay comprise a disc mounted for rotation on the same shaft as thestorage device and carrying a series of sensible indi-cia which areequal in number to the desired number of timing signals to be producedfor one rotation of the storage device and spaced at substantiallyuniform intervals around a circular track on the disc, and sensing meansadapted and arranged toderive electrical signals from the passage ofsaid indicia past it during rotation of the disc. The indicia may forexample be discontinuities in the magnetic or electric properties of thedisc or a surface layer thereof, these being sensed by suitable magneticor electric sensing means, or discontinuities in the light transmittingor reflecting properties of the disc or a surface thereof, these beingsensed by a photo-electric sensing meansarranged to operate incombination with a suitable light source.

Each secondary clock track may be recorded on a different track fromthat on which the primary clock track or the preceding secondary clocktrack is recorded, or if the method of recording is such that anythingpreviously recorded on the part of the storage device on which recordingis taking place, is automatically erased, the secondary clock tracks maybe recorded over the primary clock track or the preceding secondarytrack, as the case may be.

The present invention also provides apparatus for use in the productionof a magnetic storage device having a clock track recorded on it, theapparatus comprising means for recording a primary clock track on thestorage device, means for reading signals from a track on the storagedevice, means for recording signals on the same or another track on thestorage device and signaltr-anslating means for coupling the reading andrecording means and including means for reducing amplitude and/ orfrequency modulation of signals :fed to it.

The means for recording a primary clock track may, in the case of arotary storage device, comprise a signal generator which is mechanicallysynchronised to the rotation of the storage device and means forrecording signals produced by said generator on a selected track on thestorage device.

Examples of methods according to the present invention of producing amagnetic storage device which has a clock track recorded on it, andapparatus for use in the production of such magnetic storage deviceswill now be described by way of example, with reference to theaccompanying drawing in which:

FIGURE 1 is a schematic diagram of parts of the apparatus FIGURE 2 is ablock diagram of circuits forming part of the apparatus, "and l FIGURES3A and 3B show detailed circuit diagrams of parts of one form of theblock circuit of FIGURE 2.

In all methods according to the invention, the recording of a clocktrack is carried out in at least two stages.

track and submitting the signals derived by so doing to amplitudelimitation and frequency filtering.

In the particular case to be described with reference to the drawings,the storage device is a magnetic drum and the signals recorded to formthe primary clock track are derived from a signal generator which ismechanically synchronised to the rotation of the drum.

Referring now to FIGURE 1 of the drawings, there is shown a master clockdisc 1 forming part of the signal generator which disc 1 is mounted onthe same shaft 2 as a magnetic storage drum 3'. One end of the shaft 2is coupled by a releasable coupling 8 to the output shaft of a motor 30.The disc 1 is of glass.

A part 1a of the disc 1 is shown in FIGURE 1 as it appears when viewedaxially and it will be seen from it that the disc 1 carries on one facea circular array of alternately opaque and transparent lines 6 which canbe produced for example by thewell known technique of photo-etching. Thenumber of transparent 6 is made equal to the number of timing signalsrequired per revolution of the drum 3 from a clock track which isrequired to provide a timing signal for each signal element position onthe signal tracks. Light from a diffused illumination source 4 passesthrough a slotted screen 5, an optical system 7 and a part of the disc 1where it carties the lines 6, to a photo-electric cell 9, thearrangement being such that, when the disc 1 rotates in operation, thecell 9 is alternately substantially fully illuminated by the source 4and then not illuminated. If the lines 6 are very fine, some diflicultymay be experienced in setting up the optical system and it has beenfound, preferable in such a case to interpose in the light path a screencarrying a strip which is the negative of a photograph of a part of thearray. If the strip aligned so that lines on it are at a slight angle tothose on the disc which are illuminated, so-called Moire fringes areformed which traverse the cell 9 in synchronism with the passage of thelines 6 and give rise to the required alteration of illumina tion andnon-illumination of the cell 9.

Taking a particular case in which the drum 3 is required to rotate inoperation at 6,000 r.p.m. and to have a clock track from which a clockpulse train having a repetition frequency of 250 kc./s. can be produced,the clock track must have 2,500 recorded pulsed on it and the number oflines 6 in the array on the disc 1 will therefore also be 2,500.

It will be appreciated that the disc and sensing arrangement may takeother forms than that described above. For example, an opaque slotted ortoothed disc may be sensed photoelectnically. Alternatively, magneticsegments on a non-magnetic disc may be sensed by a magnetic sensinghead, or conducting segments on a nonconducting disc may be sensed by anadjacent probe sensitive to changes in capacity.

During recording of the primary clock track, 'the shaft 2 is rotated ata reduced speed of approximately 100 r.p.m. The shaft 2 may for examplebe rotated by hand after dis-connecting the coupling 8, or the coupling.8 may be replaced by a reduction gear so that the shaft 2 may be drivenat the reduced speed by the motor 30. This speed of rotation was chosenbecause it produced an output signal having a repetition frequency ofabout 25 kc./s., which was 'within the optimum operating region of theparticular type of photo-electric cell which was employed.

The drum 3 is shown in 'FIGURE 1 as having two tracks 17 and 18 withwhich are associated magnetic transducing heads 15 and 16 respectively,but it will be appreciated that many more may be provided, switchingmeans being provided to connect them as required to signal input oroutput paths in known manner.

Turning now to FIGURE 2 which shows a block circuit diagram of one formof the electronic apparatus which may be used with the apparatus ofFIGURE 1 in carrying out methods according to the invention, thephotoelectric cell 9 is shown coupled to an amplifier 10, the output ofwhich is fed through an amplitude clipper circuit 11 to a switch 31.With the switch 31 in the position shown in FIGURE 2, the output of theclipper circuit 11 is passed through a phase splitter 12 and a push-pullamplifier 13 to a switch 32. With the switch 32 in the position shown inFIGURE 2, the output of the push-pull amplifier is applied to an outputtransformer 14, designed for operation at 25 kc./s., and thence to thetransducing head 15 of FIGURE 1 associated with the track 17 of the drum3.

Thus with the disc 1 and drum 3 rotated as described at about rpm, thepulse signals developed by the cell 9 as a result of the passage of thelines '6 on rotation of the master clock disc 1 are recorded by the head15 on the track 17 of the drum 3 to form a primary clock track. Anyvariation in the amplitude of the pulses generated by the photo-cell 9is reduced by the clipper circuit 11. The phase splitter 12, thepush-pull amplifier 13 and the transformer 14 are of conventional designsuch that, under normal operating conditions, the track 17 ismagnetically saturated in one direction in the absence of a pulse and inthe other direction when a pulse occurs. Thus, any signal alreadyrecorded in the track 17 is automatically erased by over-writing by anew signal. The amplifier 10 and the clipper circuit 11 are also ofconventional design.

It has been found, however, that an abrupt change in the amplitude ofthe recording current passed through a head recording on a storagedevice such as the drum 3 tends to produce an unwanted phase shift inthe recorded signal. To avoid such shifts, the amplifier 13 is initiallycut off by suitable adjustment of a variable bias control resistor 34.With the disc 1 and the drum 3 rotating all the time, the resistor 34 isslowly adjusted to reduce the bias until the amplifier 13 is operatingnormally so that recording can take place, and is then slowly returnedto the cut off position. The adjustment of the resistor 34 may beeflfected manually, or it may be controlled by a. cam (not shown) driventhrough reduction gearing (not shown) from the shaft 2.

For the second and further stages of the method for which the disc 1 ispreferably removed from the shaft 2, the switch 19 can be set to coupleeither of the heads 15 or 16 to the input of a reading amplifier 20. Theoutput of the amplifier is coupled to the input of a-cascade of filterand clipper circuits 21, 22, 23 and 24, of which the filter circuits 21and 23 are designed to reduce any frequency variation in a signalnominally of 250 kc./s. frequency and the clipper circuits 22 and 24 tolimit the amplitude of such a signal so as to reduce any spuriousamplitude modulation. The output of the second clipper circuit 24 isconnected through a switch 33, when in the position shown in FIGURE 2,and switch 31 which is changed over from the position shown in FIGURE 2,to the input of the phase splitter 12. For the second stage, theswitches 32 and 36 are also changed over so that the output of theamplifier 32 is fed through a trans- I ary clock track. In doing reasonas it is during the recording of the primary clock track.

It was found to be satisfactory to use parallel resonant circuits as thefrequency selective elements in the filter circuits 21 and 23. However,other frequency selective elements, such as bridged-T circuits, mayequally well be used. Conventional diode amplitude limiting circuits orlimiting amplifiers may be used for the clipper circuits 11, 22 and 24.

After completing the recording of a secondary clock track in the track18, the switches 19 and 26 may be changed over and the second stagerepeated. The head 16 then reads the secondary clock track and producessignals which are fed through a chain of circuits 2144 already describedto drive the amplifier 13 to energise the head 15 to record a furthersecondary clock track in track 17 of the drum 3. This further secondarytrack' will be improved as compared with the first one by the action ofthe filter and clipper circuits 21-24. It will be appreciated that thesecond stage may be repeated several times in this way, the secondaryclock track recorded during one stage being read to provide the inputsignals to the amplifier 20 during the next stage. In one particularcase, it was found that four recordings of a secondary clock track weresufi'icient to reduce the frequency deviations of the clock pulse trainderived from the final one to less than 0.4% of that of the trainderived from the primary clock track which was a sufficient degree ofuniformity for the apparatus in which the storage drum 3 was to be used.The number of repetitions of the second stage required to produce agiven standard of uniformity depends upon the accuracy of the primarytrack recorded from the disc 1 and the efficiency of the filter andclipper circuits 21-24.

If theswitch 33 is set to the position other than that shown in FIGURE2, the output of the clipper circuit 24- is fed to a frequency divider27 which operates to produce an output signal once for every nth one(where n is an integer that can be determined according to requirements)of the signals applied to it. The output of the divider 27 is fed to apulse generator 28 to produce for each output signal from the divider 27an output pulse" of the required form which is fed to an amplifier andtransformer circuit 29, the amplifier of which is similar to theamplifier 13 and has a bias control resistor 35, and the transformer ofwhich is designed for operation at the frequency of the output from thedivider 27. The output from the amplifier and transformer circuit 29 isfed to the head 16 for recording in the track 18 of the drum 3.

This part of the circuit may be employed to produce a clock track foruse in producing a timing signal for each of a number of equal groups ofsignal element posi- .tions on one of the signal tracks, for example atiming signal for each word recorded on one of the information tracks ofa storage drum of a digital computer. Such a clock track may be recordedon the track 18 of the drum 3.from a secondary clock track on the track17,

the switching being set as in FIGURE 2 and the switch 33 changed over.Then with the drum 3 running at its normal speed, the secondary clocktrack is read and the signals so produced are passed after the usualfiltering and amplitude limitation to the frequency divider 27. This maybe such for example as to produce an output signal for every sixteenthor thirty second one of the signals fed to it, so that a clock track isrecorded on the track 18 which has a signal for every group of sixteenor thirty two signal element positions on the track. During thisoperation the bias control resistor 35 is operated in the mannerdescribed for the resistor 34.

The frequency divider may consist of a known multivibrator or blockingoscillator circuit which operates at .6 the required output frequencyand is synchronised by the clock pulses, or a chain of binary countingstages. The frequency division ratio is determined according to theparticular requirements of a given case. The pulse generator 28 may be amono-stable trigger circuit for example.

It is also possible to substitute a frequency multiplier for thefrequency divider 27 so as to record a clock track for producing timingsignals having an even higher repetition frequency than those producedby the signals derived from a secondary clock track recorded on the drum3. It is also possible to employ a frequency multiplier when recording asecondary clock track from signals produced by reading the primary clocktrack or in recording the primary clock track from the signals producedby sensing the disc 1.

In addition, a clock track providing one output pulse per rotation ofthe drum 3 may also be recorded on a eparate track on the drum 3. Thisclock track may be recorded by running the drum 3 with an erasing headfor the track concerned energised until there is nothing recorded on thetrack. The drum 3 is thenstopped with the desired pulse recordingposition beneath a recording head which is then energised by passing adirect current through it such as to produce magnetic saturation of thedrum surface. If this revolution marker clock track is recorded first,the pulses produced by reading it, may be utilised to synchronise anoscilloscope for monitoring the signals produced from the other clocktracks which are subsequently recorded.

'or by sensing other forms of discontinuities on a similar disc, it willbe appreciated that any other form of signal generator of suitablefrequency and suitably synchronised to the rotation of the drum 3 may beprovided. Similarly whilst certain block circuits have been describedwith reference to FIGURE 2, their function may equallybe carried out byother circuit arrangements suited to particular circumstances. Basicallyall that is required is the filtering and amplitude limiting of thesignals derived from the primary clock track or a secondary clock trackbefore using them to record a or another secondary clock track.

The particular method described was for recording a clock track on amagnetic storage drum and it will be appreciated that it may be readilyadapted for use in recording clock tracks on other forms of rotary (orotherwise cyclically operating) magnetic storage devices, for examplemagnetic disc storage devices. The same basic method may also be usedfor recording clock tracks on magnetic tapes or bands, a primary clocktrackbeing recorded first from some external source, and signalsproduced from the primary track being recorded, after filtering andlimiting, to produce a secondary clock track.

As mentioned above, practical realisations cf the block circuit shown inFIGURE 2 may take many forms. FIGURES 3A and 3B show detailed circuitdiagrams of parts of one such that was developed for use in theparticular case already mentioned in which the drum rotates at 6000 rpm.and is required to have a clock track from which a clock pulse trainhaving a repetition rate of 250 kc./s. can be produced. In addition, inthis case, the drum 3 is intended for use in a digital computer in whichsignals are recorded on the signal (or information) tracks are in binaryform, a binary one being recorded as a positive pulse followed by anegative pulse and a binary zero as a negative pulse followed by apositive pulse. The pulse separation in either case is two microseconds.In known manner, the heads used with this drum for recording in thisway, are head having an earthed center tap on their windings. The twohalvesof the head Winding are connected in the anode circuits of likevalves so that a current flows in either half when the correspondingvalve conducts, the currents in the two halves giving rise to oppositemagnet-isation of the drum surface. To record a binary signal elementthe two valves are rendered conducting in turn at the required timeinterval and in the order required by the nature of the element to berecorded.

To record a clock track to provide a clock pulse for every signalelement position, a series of binary ones is recorded at every signalelement position on the track selected for this purpose.

It will be appreciated that this method of recording using centre tappedheads necessitates appropriate variation of what may be termed therecording circuits of FIGURE 2, i.e. the phase splitter 12, the pushpull amplifier 13 and the output transformers 1-4 and 29. The sameapplies to the amplifier and transformer circuit 29 used when recordinga word maker clock track for example. The output transformers are infact not required and the phase splitter and push-pull amplifier have tobe replaced by other circuits which are described below.

FIGUTE 3A shows a combined filter and clipper circuit which incorporatesswitches to enable it to be used either as the clipper circuit 11 alone,when recording the primary clock track, or "as the filter and clippercircuits 21-24, when recording a secondary clock track.

Referring now to FIGURE 3A, the coaxial input socket 40 of the circuitmay be coupled to the output of either a pro-amplifier (not shown)associated with the photoelectric cell 9 or a reading pro-amplifier (notshown) which can be coupled to receive the output from either of theheads and '16, the selection being made according to the particularstage of the clock track recording method that has been reached. Thepro-amplifiers are of conventional form. The centre connection of socket40 is connected through resistors 41 and 42 to the control grid of adouble triode valve 43 connected as a conventional limiting and squaringamplifier. Between the common terminals of the resistors 41 and 42 andearth, are connected in series a parallel resonant circuit 44, theresonant frequency of which is 250 kc./s., and a resistor 45. Theresistances 41 and 45 are both large compared with the impedance of thecircuit 44 other than at or near its resonant frequency. A switch 46a isprovided which, when down, (down and up are used here with reference tothe posit-ions shown in the drawing) short circuits the resistors 45leaving the circuit 44 connected across the grid circuit of the firsthalf of the valve 43 and earth. In this condition, the circuits act as afilter efl'fectively passing only signals the frequency of which is 250kc./s. With the switch 46a up, the circuit of the valve 43 ads as asquaring and limiting amplifier to the signals of frequency 25 kc./s.which are applied to it. The circuit of the valve 47 is identical and isfed from the anode circuit of the second half of the valve 43. Theswitch 46b is ganged to switch 46a and the two are operated to be upduring the second and further stages of a recording, so that thefiltering action is obtained, and to be down during the first stage. Ineither case amplitude limiting and squaring takes place such as toreduce any amplitude variation in the signals passed through thecircuit. Two output terminals 48 and 49 are connected to the anode ofthe second half of the valve 47. The remaining parts of the circuit areidentical and will not be described. The output appearing in operationat the terminals 48 and '49 is a substantially square wave having afrequency nominally of 250 kc./s. or 25 kc./s. depending on the stage ofthe method that has been reached.

FIGURE 3B shows a circuit diagram of pulse generating and amplifyingcircuits which can be used with the circuits of FIGURE 3A, to operateearthed centre tap type recording heads. The pulse generating circuitsare arranged so that they can be switched to operate as a 8 frequencydivider, thus eliminating the need for the separate chain of circuits27-29 of FIGURE 2.

Referring now to FIGURE 3B, the circuit shown has two input terminals 50and 51 which are connected to the output terminals 48 and 49 of FIGURE3A respectively. In operation, therefore, the input to the circuits ofFIG- URE 3B will be substantially square waves. Considering the inputterminal 50 first, this input is differentiated by a differentiatingcircuit consisting of a capacity 52 and a resistor 53, the result beingapplied to the control grid of a triode thermionic valve 54. As anegative bias is applied from the l7 volt supply line 55, only theposiitive pulse of the diflerentiated Waveform is effective, producing anegative pulse at the anode of the valve 54. This is applied to thecathode of a triode valve 56 which is connected as a blockingoscillator. With the switch 57 in the up position, the blockingoscillator operates to produce one output pulse for each pulse appliedto it, the length of its recovery time being determined largely by themagnitudes of the capacitor 58 and the resistor 59. Negative-goingoutput pulses are therefore applied to the control grid of a pentodevalve 60 which is connected conventionally as an amplifier. Positivegoing output pulses, squared and limited in amplitude by the diodes 61,are applied from the anode circuit of the-valve 60 to the control gridof a pentode valve 62, one of a pair, the operation of which isdescribed below. A second output, of smaller amplitude is applied overthe line 63 to one pole of a switch 64a in a part of the circuit not yetdescribed.

The valve 62, with a similar valve 62 are output amplifiers for passingthe appropriate currents through the half windings of a centre-tappedrecording head, to record a binary element. As will be described, thevalve 62 during the recording of a secondary clock track, receives forthis purpose a positive pulse at its control grid two microseconds aftereach pulse applied to the control grid of the valve 62. The two endterminals of the head winding (not shown) are connected by screenedcables to the coaxial output sockets 66 and 66 the centre'connections ofwhich are connected through a double pole switch '67, when closed, tothe anodes of the valves 62 and 62 These are in fact conventionalamplifiers with their anodes earthy and their cathodes connected througha common load resistor 68 to a volt negative supply line. The controlgrids of the .tWo valves 62 and 62 to which input pulses are applied,are connected through resistors 69 and 69 to the variable tapping of apotentiometer 70 forming part of a resistance chain connected betweenearth and a negative 300 volt supply line. Diodes 71 and 71 areconnected across the resistors 69 and 69 respectively. The control gridsof the valves 62 and 62 are thus, in the absence of a signal pulse,maintained at a potential determined by the setting of the potentiometer70. This is manipulated during recording of a clock track in the mannerdescribed previously with reference to FIGURE 2 concerning the biascontrols 34 and 35, so that the valves 62 and 62 are at first cut rightofi, even during the application of a pulse to their control grids, aresubsequently brought gradually to the point where they are biased sothat each pulse applied to the control grid .of either valve 62 01 62causes the required current pulse to flow in the corresponding half ofthe head winding, and are then gradually cut off again after recordinghad finished. The centre tap of the head winding is connected by ascreened cable to the earthed coaxial soclnet 65.

To provide the delayed pulses required :for application to the controlgrid of the valve 62 two microseconds after each pulse applied to thevalve 62 the switch 64 which is to the right (as shown in FIGURE 3B)during recording of a secondary clock track, applies the positive pulsesreceived from the anode circuit of the valve 60 to the control gridcircuit of a triode valve 72 which is connected as a cathode follower.The cathode load of this includes a .two microsecond delay line 73, theoutput of which is applied to the control grid of a triode amplifiervalve 74. An output of negative pulses from the anode of the valve 74 isapplied to the cathode of a triode valve 56 which is connected as ablocking oscillator similarly to the valve 56, the switch 64b being tothe right in the case of recording a secondary clock track. The circuitconstants of the oscillator formed by the circuit of valve 56 withswitch 64b to the right are identical with those of the circuit of valve56 when the switch 57 is up, as it is during recording of a secondaryclock track. As in the case of the valve 56, a negative going pulseoutput is taken from the anode circuit of the valve 56 inverted in theamplifier formed by the circuit of the valve 69 and applied as apositive going pulse to the control grid of the valve 62 These pulseswill be, as required, delayed by two micro-seconds on the correspondingpulses applied to the control grid of the valve 62 owing to the delayintroduced by the delay line 73.

If frequency division is required, for recording of a work marker clocktrack, the switch 57 in the circuit of the valve 56 is changed to itsdown position where the high resistance chain formed by a variableresistor 75 and a fixed resistor '76, alter the time constants of itsrecovery, such that it responds only to every nth one of the pulsesapplied to it, n being an integer which can be varied by adjustment ofthe resistor 75. 'In this case, the circuit for the generation of pulsesfor application to the valve 6-2 formed by the circuits of valves 72,74, S6 and 66 will also receive a pulse only for every nt-h one of thepulses applied to the valve 56, as it is fed, the switch 64a still beingto the right, from the anode circuit of the valve 6% When recording aprimary clock track, a difierent longer time spacing of the pairs ofpulses applied to the valves 62 and 62 is required, if the recordedpulses are to give the required output when the drum 3 is run at 6000rpm. during recording of a secondary clock track from the primary one.It will be remembered that the output from the circuit of FIGURE 3A is asquare wave, the positive going crossovers of which are effective totrigger the valve 56. The negative going crossovers of the 25 kc./s.square wave derived from the master clock disc 1 are at the correct timespacing vfrom the positive ones, and it is from these that the pulsesapplied to the valve 62 during recording of a primary clock track arederived. To this end, the switch 65a is changed over to the left.disconnecting the grid circuit of valve 72 from the line 63 andconnecting it instead to the anode circuit of an amplifier triode valve77. The circuit of this is arranged so that it is normally bottomed andits control grid circuit includes a differentiating circuit formed by acapacitor 78 and a resistor 79. The input terminal 51 is connected tothe output terminal 49 of the circuit of FIGURE 3A and thus receives thesquare wave output from that circuit. Owing to the bottomed condition ofthe valve 77, only the negative going pulses formed on diiferentiationare passed by the valve 77 as positive going pulses to the control gridcircuit of the cathode follower valve 72. These pulses correspond to thenegative going crossovers of the square wave and are eifective totrigger the blocking oscillator valve 56 in the same way as the pulsesreceived from the valve 60 during recording of a secondary clock track.The two microsecond delay introduced by the delay line 73 is smallenough to be negligible in this case and pulses with the required timingare thus applied to the control grid of the valve 62 In recording aprimary clock track, the switch 64b is in its left hand position thusdisconnecting the capacitor 58 and the resistor 59 and connecting bothblocking oscillator circuits to use the same capacitor 58 and resistor59. This is done to prevent spurious triggering of either after firingof the other which has been found to occur due to pick-up between thecables connecting the 1d circuit of FIGURE 33 to the head winding towhich-it is connected.

The circuits described with reference to FIGURES 3A and 313 weredesigned for use in a particular case and it will be appreciated thatthey are in no way essential to carrying out methods according to thepresent invention.

I claim:

1. Apparatus for recording synchronising signals on a track of amagnetic storage device operating cyclically at a constant ratecomprising reading means operative during one cycle of the storagedevice in response to each of a series of signals recorded atapproximately equal spacing in said track to produce an electricalsignal, said electrical signals being produced approximately at aconstant frequency; means for temporarily storing each of saidelectrical signals for a predetermined duration and having an inputcircuit connected to the reading means and having an output circuit;said temporary storage means including a filter network operative toattenuate, relative to signals of said constant frequency, all othersignals to cause signals in said output circuit to be more nearlyequally spaced than the electrical signals; and recording meansoperative in response to the signals in the output circuit to record thelike number of synchronising signals in said track.

2. Apparatus for recording synchronising signals in a track of amagnetic storage device operating cyclically at constant rate comprisingmeans for recording a train of nearly equally spaced first signals in afirst track of the storage device; means responsive, during one cycle ofthe storage device, to said recorded train of first signals to generatea like train of electrical signals having substantially a constantfrequency; signal amplitude limiting means; a filter network operativeto attenuate, relative to signals of said constant frequency, all othersignals; means operative to feed said train of electrical signalsthrough said amplitude limiting means and said filter network insuccession to produce a train of synchronising signals; and means forrecording said train of synchronising signals in a second track of thestorage device.

3. Apparatus for recording a predetermined number of synchronisingsignals on a track of rotor magnetic storage device comprising a discmounted for rotation in synchronism with the storage device; a series ofsensible indicia equal in number to said predetermined number ofsynchronising signals and spaced at substantially uniform intervalsaround a circular track of the disc; sensing means responsive to thepassage of said indicia past the sensing means during a revolution ofthe storage device to produce a train of first signals equal in numberto said predetermined number; a first track of said storage device;first recording means responsive to said first signals to record saidfirst signals at substantially equal spacing in said first track;driving means operable to rotate the storage device at a constantinvariable speed; reading means operative, during rotation of thestorage device at said constant speed, in response to said recordedfirst signals to generate a train of second signals at a substantiallyconstant frequency and equal in number to said predetermined number; asecond track on said storage device; second rcording means operativelycoupled to said second track; circuit means including a filter networkoperative to attenuate, relative to signals of said constant frequency,all other signals; and having an input circuit connected to said readingmeans and an output circuit connected to said second recording means.

4. Apparatus for recording uniformly spaced synchronising signals on atrack of magnetic storage device operating cyclically at constant ratecomprising first and second tracks on the storage device; means forrecording a train of substantially uniformly spaced signals in saidfirst track; first signal transducing means operatively coupled to saidfirst track and responsive to said train of signals during a first cycleof the storage device to generate a A 1 l 7 like train of electricalsignals substantially at a constant frequency; circuit means having aninput terminal and an output terminal and including a filter networkconnected between said input and output terminals and opducin-g means tothe input terminal and the first transducing means to the outputterminal; said first transducin-g means being operative during saidfurther cycle to record in said first track signals from said outputtermierative to attenuate, relative to signals of said constant 5 Halfrequncy, all other signals; second signal transducing means operativelycoupled to said second track; and switching means operative during saidfirst cycle to connect the first transducing means to the input terminaland the second transd-ucing :means to the output terminal and operativein a further cycle to connect the second trans- References Cited in thefile of this patent UNITED STATES PATENTS

3. APPARATUS FOR RECORDING A PREDETERMINED NUMBER OF SYNCRONISINGSIGNALS ON A TRACK OF ROTOR MAGNETIC STORAGE DEVICE COMPRISING A DISCMOUNTED FOR ROTATION IN SYNCHRONISM WITH THE STORAGE DEVICE; A SERIES OFSENSIBLE INDICIA EQUAL IN NUMBER TO SAID PREDETERMINED NUMBER OFSYNCHRONISING SIGNALS AND SPACED AT SUBSTANTIALLY UNIFORM INTERVALSAROUND A CIRCULAR TRACK OF THE DISC; SENSING MEANS RESPONSIVE TO THEPASSAGE OF SAID INDICIA PAST THE SENSING MEANS DURING A REVOLUTION OFTHE STORAGE DEVICE TO PRODUCE A TRAIN OF FIRST SIGNALS EQUAL IN NUMBERTO SAID PREDETERMINED NUMBER; A FIRST TRACK OF SAID STORAGE DEVICE;FIRST RECORDING MEANS RESPONSIVE TO SAID FIRST SIGNALS TO RECORD SAIDFIRST SIGNALS AT SUBSTANTIALLY EQUAL SPACING IN SAID FIRST TRACK;DRIVING MEANS OPERABLE TO ROTATE THE STORAGE DEVICE AT A CONSTANTINVARIABLE SPEED; READING MEANS OPERATIVE, DURING ROTATION OF THESTORAGE DEVICE AT SAID CONSTANT SPEED, IN RESPONSE TO SAID RECORDEDFIRST SIGNALS TO GENERATE A TRAIN OF SECOND SIGNALS AT A SUBSTANTIALLYCONSTANT FREQUENCY AND EQUAL IN NUMBER TO SAID PREDETERMINED NUMBER; ASECOND TRACK ON SAID STORAGE DEVICE; SECOND RECORDING MEANS OPERATIVELYCOUPLED TO SAID SECOND TRACK; CIRCUIT MEANS INCLUDING A FILTER NETWORKOPERATIVE TO ATTENUATE, RELATIVE TO SIGNALS OF SAID CONSTANT FREQUENCY,ALL OTHER SIGNALS, AND HAVING AN INPUT CIRCUIT CONNECTED TO SAID READINGMEANS AND AN OUTPUT CIRCUIT CONNECTED TO SAID SECOND RECORDING MEANS.